In layers of ferromagnetic transition metals such as Ni, Fe or Co and alloys thereof, an electric resistance of a layer may depend on the size and direction of a magnetic field permeating the layer's material. The effect that occurs with such layers is called anisotropic magneto-resistance (AMR), or the anisotropic magneto-resistive effect. Physically it is based on different scattering cross sections of electrons with different spins and their spin polarity in the D band. Therefore, these spinning electrons are referred to as majority and minority electrons. A thin layer of such a magneto-resistive material, with magnetization in the plane of the layer, is generally provided for use in magneto-resistive sensors. Change in the electric resistance of the layer, caused by rotation of the magnetization plane in response to a direction of an electrical current passing through the layer, may amount to a plurality of percentage of the normal isotropic (i.e., ohmic) resistance of the layer.
Magneto-resistive layer systems containing a plurality of ferromagnetic layers arranged in a stack and separated from each other by nonmagnetic intermediate layers, with the magnetization of each preferably in the plane of the layer, are known. The thickness of each individual layer is selected to be much smaller than the mean free path length of the conduction electrons. In such multilayer systems, a giant magneto-resistive effect or giant magneto-resistance (GMR) may also occur in addition to the above-mentioned anisotropic magneto-resistive effect (AMR). Such multilayer systems are described in European Patent No. 0 483 373 A and German Patents Nos. 42 32 244 A, 42 43 357 A and 42 43 358 A. Such a GMR effect is based on different intensities of the scattering of the majority and minority conduction electrons at the interfaces between the ferromagnetic layers and the adjacent nonmagnetic intermediate layers and on the scattering effects within these layers. The GMR effect is an isotropic effect and may be much greater than the anisotropic effect (AMR). As such, the GMR effect may assume values amounting to at least 70% of the normal isotropic resistance.
With a first type of such a multilayer system having a GMR effect, adjacent magnetic layers have a magnetically opposite, or antiparallel, orientation without an external magnetic field because of their mutual coupling. This orientation can be converted to a parallel orientation by an applied external magnetic field. On the other hand, a second type of GMR multilayer system has a bias layer, or a bias layer portion, that is magnetically harder than a (magnetically softer) measurement layer. These two layers are mutually isolated magnetically by a nonmagnetic intermediate layer. Without an external magnetic field, the magnetizations of the two magnetic layers have some relationship to each other, e.g., antiparallel. Under the influence of an external magnetic field, the magnetization of the magnetically soft measurement layer is oriented according to the direction of the field, whereas the orientation of the magnetically harder bias layer remains unchanged. The angle between the directions of magnetization of the two magnetic layers determines the resistance of the multilayer system. With a parallel orientation (i.e., same direction), the resistance is low, and with an opposite orientation the resistance is high. This fact is utilized in corresponding magnetic field sensors.
A sensor device with four such magnetic field sensor elements wired to form a bridge circuit is described in German Patent No. DE 44 27 495 A. In each branch of the bridge, the two sensor elements have magnetizations of their bias layer portions oriented at least essentially opposite to each other. These sensor elements are thus of the second type mentioned above.
With such multilayer systems having GMR sensor elements of the second type, the electric resistance can be broken down into two components, namely a magnetic field-sensitive component .DELTA.R and a fundamental resistance R.sub.0 that is not sensitive to the magnetic field influence. In sensitive systems, the field-sensitive component .DELTA.R typically amounts to 5% to 30% of the fundamental resistance R.sub.0. In the electronic analysis of the sensor signal, fundamental resistance R.sub.0 acts as an interfering offset voltage. It has also been found that fundamental resistance R.sub.0, in particular, depends on an operating temperature T. For example, at room temperature T.sub.a the fundamental resistance R.sub.0, which is characterized by a strongly temperature-dependent contribution of the photons, is approximately proportional to T/T.sub.a and amounts to approximately 0.1% per degree Kelvin. Such a change is undesirable for most applications. For this reason, individual magneto-resistive sensors have been provided with an additional sensor, or a bridge design with four sensors.
With the known bridge circuit having four GMR sensor elements whose magnetic layer systems are of the second type having a magnetically harder bias layer portion and a magnetically softer measurement layer, only an angle range of 180.degree. with regard to one component of an external magnetic field can be detected. Consequently, for the full angle detection range of 360.degree. it is therefore necessary to provide another sensor device with a bridge circuit, where the reference axes of the two bridges are normal to each other. In other words, two sensor devices, whose magnetizations are oriented normal to each other, are required for the magnetic field information, and sensor devices with bridge circuits are required for offset voltage compensation. However, this means that a total of eight sensor elements are needed, which would take up a large amount of area and would have to be balanced with respect to each other.